Thermostat



Dec. 15, 1942. w. A. SAUL 2,305,446

THERMOSTAT Filed Sept. 25, 1940 3 SheetS-Sheel; l

W. A. SAUL Dec. l5, 1942.

THERMOSTAT File`d Sept. 25, 1940 5 Sheets-Sheet 2 MHG. I3.

. FIG. I4.

FIGS.

W. A. SAUL THERMOSTAT Dec. l5, 1942.

Filed Sept. 25, 1940 3 Sheets-Sheet 3 www www? Patented Dec. 15, 1942 T orifice THERMOSTAT Waldo A. Saul, Lexington, Mass., assignor to Metals & Controls Corporation, Attleboro, Mass., a corporation of Massachusetts Application September 25, 1940, Serial No. 358,199

1s claims. (ci. 20o-113) This invention relates to thermostats, and more particularly to thermostats containing composite thermostatic elements wound in the form of a helix such as shown and described in Parsons United States Patent 2,121,259, dated June 2l, 1938.

Among the objects of the invention may be noted the provision of a simple and compact thermostat embodying a thermostatic element such as shown in said Persons patent, which operates automatically but with manual re-setting; the provision of such a thermostat which may be made sensitive to rates of change of temperature; the provision of such a 4thermostat which in certain useful respects, is not affected by ambient temperature changes; the provision of a thermostat of the class described which operates with a snap action, thereby eliminating or decreasing arcing between contacts; and the provision of a thermostat of the class described which is in one form trip-free of the re-setting mechanism. Other vobjects will be in part obvious and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction, and arrangements of parts which will be exemplied in the structures hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings, in which are illustrated several exemplary embodiments of the invention,

Fig. lis a plan view of one type of thermostat made in accordance with the present invention;

Fig. 2 is a front elevation of the Fig. 1 thermostat;

Fig. 3 isa cross-section taken along the line' 3-3 of Fig. l;

Fig. 4 is a left end elevation of the Fig. 1 thermostat;

Fig. 5 is a section taken along the line 5 5 of Fig. 2;

Fig. 6 is a section taken along the line 6-5 in Fig. 2;

l Fig. 7 is a section taken along the line 1-1 of Fig. 2;

Fig. 8 is a section taken along the line S-- in Fig.` 2;

Fig. 9 is alplan view of another type of thermoin Fig. 10; and,

Fig. 17 is a section similar to Fig. 11, but showing a still different embodiment of the invention.

Similar reference characters indicate corresponding parts throughout the several views of the drawings.

The present invention refers to composite thermostat elements of the type shown and described in said Parsons Patent No. 2,121,259, and to thermostats embodying these composite elements. An element of this type may be made by iirst winding a narrow strip of composite thermostatic metal, such as bimetaLinto a helical form lllaving'a relatively small diameter. This minor helix is then wound as a second or major helical form of relatively large diameter, thus forming a compound or double helical thermostatic element, or a peripherally continuous, operating member, as described in said Parsons Patent 2,121,259. The composite metal, or bimetal helix, or spring, so formed, will expand or contract lengthwisewith change of temperature. The amount of movement for a given temperature change is controlled by the relation between the various dimensions of the element, and the direction of movement is controlled by the direction of the winding of the two helices and the relative position' of the high and low expansion sides of the bimetal.

Referring to the drawings, Fig. 1 shows a thermostat'l, embodying helices 2 and 3 of the type described above. The end 4 of thermostatic element 2 is attached tov a member 5, provided with a terminal screw 6. The other en'd l of thermostatic element 2 is attached to member 8, for example, by welding, as shown at 9 (see Fig. l). Member 8 is held in desired position on shaft IIJ by screw il. Shaft I0, it will be noted, extends through the center of the helix 2. On one end of shaft iD is xed a manual re-setting means, such as the pointer l2. This may be in the formof an oil-on switch, such as sliowr.-

Fig. 4. The shaft lil, extends not only through the thermostatic element 2, but also through the thermostatic element 3'. The two elements are mounted approximately concentrically and separated by insulator 58 as well as by member l.-

Insulator 58 is slidably mounted on shaft I0. Shaft I is slidable endwise in its bearings in the base I, carrying with it the member 8. Member 3 has a projection .I3 upon which is mounted a contact button I4. This contact button I4 cooperates with another contact button I5, mounted upon memberA I6. Member I6 is attached to thermostatic element 3 at I1. Also attached to member I6 at I1 is a non-heating electrical conducting wire I8. Wire I8 is shown mounted axially within the small helix. This wire is connected at its opposite end I9 to member 20. Thermostatic element 3 is also attached to member 20 at 2|.

The portion I3 of member 8 is positioned to rest upon a latch 22 when the thermostat is in the Fig. 2 position. This latch is mounted upon a sliding support 9| slotted :as at 83 to provide for adjustment under a set screw 95. Adjustment is parallel to the axial movement of shaft I0.

The operation of the device is as follows: Taking Fig. 1 as the primary or starting position of the device, the portion I3 of member 8 is positioned below the latch 22. In this position the thermostatic elements 2 and 3 are substantially free of stress and the contacts I4 and I5 are open. Ihe knob I2 in this position will point to the word off as shown in dotted lines in Fig. 4. Manually rotating the setting means I2 to the on position shown in Fig, 4, rotates the shaft IU, and thereby imposes stress or torsion upon the thermostatic element 2. Such rotation likewise serves to move the catch portion I3 of the member 8 past the latch 22 so that it springingly reststhereupon, as shown in Fig. 2. This rotation also closes contacts I4 and I5, since element 3 is so positioned that for a part of its motion before portion I3 of member 8 slides up and over the latch 22, contact I4 has met contact I5 to rotate member 3 about its axis and place torsional stress therein. Contacts I4 and I5 are therefore held springingly together by the opposing torsions of the thermostatic helical elements.

Thermostatic elements 2 and 3 are so constructed and positioned in the device, that their tendencies to expand longitudinally for a given change of temperature are in opposite axial directions. Thus an increase in ambient temperature will have substantially no effect upon the longitudinal position of the portion i3 of member 8. The elements in effect buck one another to cancel out movement, that is, the tendency of thermostatic element 2 to expand toward the right in the Fig. 2 position is opposed by the equal tendency of thermostatic element 3 to expand toward the left. Contacts I4 and I5 thereby are retained in one position, and in contact, because of the previous latching and the bucking action.

v If a circuit is now established between terminals 6 and 23, the current passes from terminal 6 through member 5, to thermostatic element 2 at 4, thence from thermostatic element l2 to member 8 to contact button I4 to contact button I to member I6, and thence through both thermostatic element 3 and non-heating wire I8 to member 20 and terminal 23. Since the thermostatic element 3 is arranged in parallel with non-heating (low resistance) wire I8, passage of the current will have substantially no tendency to cause the thermostatic member 3 to expand. However the current all passes through thermomember to expand toward the right. Conditions through the circuit, is not suiiicient to cause the thermostatic element 2 to expand suiciently to bring the portion I3 of member 8 out of engagement with latch 22. However, the additional expansion caused bya deleteriously high current passing through thermostatic element 2 is sufiicient to expand the element sumciently to cause portion I3 to be moved to the right out of engagement with latch 22. As soon as this occurs, the stress in thermostatic element 2 forces member 8,

f and its portion I3 carrying contact button I4,

static member 2, and thereby tends to cause the lmounted a plate 3|.

downwardly out of engagement with contact button I5, thus breaking the circuit.

Cooling of' the device after breaking the circuit will cause the thermostatic element 2 to contract again, pulling the member 8 and its portion I3 to the left. However, there is no tendency from this action to re-introduce the torsional stress into the element 2, and as a result there is no tendency automatically to bring contacts I4 and I5 together to re-establish the circuit. This must be done mechanically by turning the knob I2 to the on" position, thus bringing the portion I3 of member 8 again on top of the latch 22, and the contact button I4 thereby into engagement with contactbutton I5.

The device is independent of ambient temperature changes, and operates with a snap action. It is also automatically re-setting to a condition for potential latching.

An alternative embodiment of the invention is shown in Fig. 9. Here a thermostat 24 has mounted thereon a thermostatic element 25, embodying a helix, such as described previously. A

shaft 26 passes through the helix, and is rotatablymounted in thethermostat. The end 21 of the shaft projects from the thermostat and has a manual resetting knob 28 mounted thereon. The thermostatic element 25 is mounted at one end on member 29, as shown at 30 (see Fig. l2). On the other end of the thermostatic element 23 is This plate is rotatably mounted on shaft 28, and has a slot 32 adapted to receive key 33, which is rigidly attached to shaft 26. This key 33 normally prevents rotation of the plate 3|. Rotation o! the entire assembly is prevented inone direction by projecting portion 34, which cooperates with a notch 3l. cut in the thermostat assembly mounting. A coil spring 36 attached at opposite ends to the frame of the thermostat and to the plate 3| respectively, operates to oppose any tendency of the plate 3| to move to the right (Figs. 9-l1) and maintains the plate 3| in operative association with the key 33 under normal conditions.4 Spring 3B also operates as a torsion spring to rotate plate 3| counter-clockwise to the Fig. 18 position where it is stopped by notch 35.

Mounted on the thermostat' frame are contact terminals 31 and 38.' These are arranged to cooperate with movable contact buttons 39 and 48, which are mounted on member 29 by means of cross bar 4I. Cross bar 4| is formed of some non-conducting substance. Attached to contact button 40 by means of a pigtail 42 is a non-heat- .button 39 to wire 48 to member 28 to thermal element 25 to plate 3| to wire 43 through pigtail 42 to movable contact 48 to stationary contact button 48 to terminal 33.

Manual re-setting button 28 mounted on shaft .the right so that itis free from the key 33.

26 has a projecting portion 49 which moves with knob 28. Thisl projection 49 is positioned to ride over projection 50 attached to the frame of the thermostat. Projection 56 is mounted on flexible plate which in turn is attached by screw 52 to the frame of the thermostat. Plate 5I is preferably made of resilient material, so that if it is desired to manually release the contacts, this may be easily accomplished by pressing upon knob 53 which projects through the thermostat frame assembly, and tends to push member 5| away therefrom, contacting it at 54.

Member 5| preferably has a projection 55 arranged to cooperate with an opening 56 in the thermostat frame assembly. The Word off may then be convenientlyv inscribed upon the projecting portion 55, and will show when the thermostat is in the Fig. 13 position. -A` projecting portion 51 may also be conveniently attached to the member 29, and so positioned that upon rotation of the knob 28 as the projection 49 passes over the projection 56 the projecting portion 51 is interposed between projection 55 and the opening 56 in the thermostat frame assembly. The word on is inscribed upon projection 51.

The operation of the device is as follows: When the thermostat is in the Fig, 13 position, so that the word 01T shows, and the projecting portion 4 9 is to the right of projection 5|), the thermostatic element 25 is substantially free of stress., Rotation of the knob 28 turns shaft 26 and thus plate 3|. Plate 3| turns (against the restraining force of spring 36) to turn the thermostatic element 25. This turns member 29 with its movable contacts 39 and 46 and brings mov- 5' able contacts 39 and 49 into engagement with stationary contacts 41 and 48. Member 29 is rotary on shaft 26 so that knob 29 may continue to rotate the shaft after the movable contacts have-engaged the stationary contacts. Such iurther rotation, which puts thermostatic element 25 under torsion, as Weil as spring 36, is necessary to bring projecting portion 49 past projection 50. Bringing the movable contacts into engagement with the stationary contacts completes a circuit, the current passing in the path described above.

The passage of normal current operates to heat the thermostatic element 25 and cause it to eX- pand toward the right in Fig. 9. This expansion is adjusted, however, so that it is not suflicient to cause the plate 3| to move suiiiciently far to However, vupon the passage of a deleteriously high current through thermostatic element 25, the eX- v pansion is adjusted to be suflicient to move the plate suiciently far to the right so that it no longer has the slot 32 therein engaging the key 33. This releases that end of the thermostatic element 25, and allows the stress stored in spring 36 by manual setting of the knob 28 to cause rotation of the member 29 assembly and moves the contacts 39 and .40 out of engagement with cont-acts 41 and 48, thus breaking the circuit. Projecting portion 34 of plate 3| cooperates with slot 35 in the thermostat frame to prevent the release of more than a part of the stress stored' in spring 36. Thus spring 36 is always torsionally stressed in a direction to tend to open the switch. It transmits' a large opening movement to the switch through plate 3| and the connected thermostatic element 2 5 even when the latter is unstressed (after plate 3| has left key 33 and contacts 39 and 40 have left contacts 41 and 48).

Re-setting of thev device is accomplished by revolution clockwise (Fig. 13), thus bringing the key-33 back into -alignment with .the slot 32 in plate 3|. If the thermostatic element 25 has then cooled sufl'ciently to contract and bring the plate 3| into engagement with the key 33, the knob 28 may then be further rotated until the portion 49 passes the projection 50 and the device will then operate as before. However, if the thermal element 25 has not cooled sufiiciently so that the plate 3l will engage the key 33, the device cannot be re-set and the circuit cannot be reclosed. The device is therefore trip-free or latch-free of the manual re-setting means.

`From the foregoing description, it will be apparent that the device will not automatically reset itself, nor will the preferred form of Figs. l-8.

An alternative form of the thermostat shown in Figures 9 to 16 is represented in Fig. 17. The thermostat in this instance has been ambientcompensated; that is, it is not affected by changes in the ambient temperature to which the device maybe subjected.

The thermostat shown has two thermostatic elements 10 and 1|. Thermostatic element 1| is similar in all respects to thermostatic element 25 in Figures 9 to 16-. Thermostatic element 1|), however, is wound in the same way as thermostatic element 1| but with the opposite side of the composite metal outward, and as a result, element 10 contracts when heated, while element 1| expands whenA heated. Elements 16 and 1| are wound so that their tendency to contract and expand respectively upon rise in temperature, is equal; that is, element 1| will expand in a longitudinal direction the same distance that element 19 will contract when both elements are subjected to the same increase in temperature. As a result, changes in ambient temperature have substantially no effect upon plate 3|. The contraction of thermostatic element 'i9 exactly counteracts and balances the expansion of ther- 1 mos-tatie element 1| thus leaving plate 3i un- 1l, its passage causing the element to expand.

affected.

Thermostatic element 1| however, is connected at 69 in series with the current which it is the purpose of the device to control, as thermostatic element 25 was so connected in the embodiment shown in Figures 9 to 16. The heating'effect of the current normally passed through the circuit is operative only upon thermostatic element This expansion, as in the case of thermostatic element 25, is adjusted so that it is not sufficient to cause the plate 3| to move suilicientlyv far to the right to be free from the key 33. Upon the passage of a deleteriously high current, however, through thermostatic element 1| the expansion is adjusted to be sufcient to move the thermostatic element 10 and plate 3| suiciently-far to the right sothat the slot 32 in plate 3| no longer engages key 33. This releases the xed end of the thermostatic elements 10 and 1|, and allows the stress in spring 36 stored by the manual setting of the knob 28 to rotate the member 29 assembly, thereby moving the contacts 39 and 40 out of engagement to break the circuit.

It will be understood of course that the adjacent ends of thermostatic elements 1E) and 1| are connected together so that the partial release of the torsion in spring 36 will rotate the movable contacts 39 and 40 out of engagement with the stationary contacts 41 and 48. Alternatively, the movable contacts may be mounted upon therturning the knob 28 through almost a complete y with contacts 41 and 48 mostatic element 10 instead of upon thermostatic element 1|. In this case it is not necessary that thermostatic elements 10 and 1| be connected together. y A,

`It Will be clear of course that the Fig. 1 to 8 method of compensating for ambient temperature change-may be applied to the Fig. 9 to 1'1 embodiments. Thus the plate 3| may be positioned between thermostatic elements 10 and 1|. Thermostatic element 10 in this case will be an exact counterpart of thermostatic element .'ll, and will expand with increase in temperature, just as thermostatic element 1| will do. Spring 36 is preferably located inside thermostatic element 1| and is attached to plate 3| and the frame 'of the thermostat as before. In this alternative, the two thermostatic elements 10 and 1| will buck one another upon increase in ambient temperature, and there will be substantially no longitudinal movement by either upon such increase in ambient temperature. Only thermostatic velement 1| is connected in series with the current, however, so that it only is afi'ected by the current which it is desired to control. A deleterious increase in this current will therefore be suiiicient to heat the thermostatic element 1| enough to push plate 3| out of engagement with key 33, and permit spring 36 to carry out the rotating action described previously, thus separating the movable contacts from the stationary contacts.

From the above it will be seen that the motivating thermostatic element of this invention consists of a strip of composite metal extended progressively about a rst minor axis to form minor coils around that axis, said minor axis being in turn extended or coiled about a second major axis to form major coils. Such a construction provides a double coil construction providing for large torsional angular strains (to accomplish angular contact action), and also to provide substantial linear axial expansion along the major axis to release the detent or latch. By means of this double coil construction, there is obtained a thermostatic element which has a large axial movement upon temperature change and which has a large torsional movement which can be utilized to close and open contacts without damaging stress being set up in the bimetal.

In View of the above, it will be seen. that the several objects of the invention are achieved and other advantageous results attained.

As many changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A thermostat comprising a helical coil axially expansible in response t temperature change composed of a strip of composite metal, a stationary'contact, a movable contact operable by said coil, means for placing the coil under stress, and alatch for preventing release of said stress before a predetermined temperature is reached.

2. A thermostat comprising two helical thermostatic members in abutment, axially expansible in opposite directions in response to temperature changes and abutting one another to cancel movement when temperature change is ambient to both thermostatic members, the abutment moving when one of the members is heated independently of the other and means for independently heating one of the members.

3. A thermostat comprising two helical thermostatic members in abutment, axially expansible in opposite directions in response to temperature changes and abutting one another to cancel movement when temperature change is ambient to both thermostatic members, the abutment moving when one of the members is heated independently of the other and means for independently heating one of the members, a second abutment between said members comprising switch contacts normally spaced, manual means for driving one Contact against the other by operation of one thermostat whereby one of the thermostatic members is tensioned, a latch for maintaining said tension, said one thermostatic member when suiilciently expanding in response to said independent heating moving the abutted contacts from the latch whereby they open.

4. A thermostat comprising two helical thermostatic members in abutment, axially expansible in opposite directions in response to temperature changes and abutting one another to cancel movement`when temperature change' is ambient to both thermostatic members, the abutment moving when one of the members is heated independently of the other and means for independently heating one of the members, a second abutment between said members comprising switch contacts normally spaced, manual means for driving one contact against the other by operation of one thermostat whereby one of the thermostatic members is tensioned, a latch for maintaining said tension, said one thermostatic member when sufficiently expanding in response to said independent heating moving the abutted contacts from the latch whereby they open, the suiiiciency of said expansion from heating being determined by overheating as distinguished from normal heating.

5. A thermostat comprising two helical thermostatic members in abutment and axially expansible in opposite directions in response to temperature changes and abutting one another to cancel movement when said temperature change is ambient to both thermostatic members, the abutment moving axially when one of the members is heated independently of the other, and means for independentlyheating said one of the members, a second and torsional abutment between said members comprising switch contacts normally spaced, manual means for driving one contact against the other by torsional operation whereby one of the thermostatic members is tensioned in torsion, a latch for maintaining the torsional tension, said one thermostatic member expanding axially in response to said independent heating to move the torsionally abutted contacts from the latch.

6. A thermostat comprising two helical composite thermostatic members in abutment and axially expansible in opposite directions in response to temperature changes and abutting one another to cancel movement when said temperature changes are ambient to both members, the abutment moving axially when one of the members is heated independently of the other, means for heating said one member independently of the other, a second and torsional abutment between said members comprising switch contacts normally spaced, manual means for driving one contact against the other by'torsiona-l operation of said one member whereby both thermostatic members are tensioned. a latch for maintaining said tension, said one thermostatic member expanding axially in response to excessiveV local heating to move the torsionally abutted contacts from the latch, said contacts automatically reopening after movement from the latch and remaining clear of the latch until the excessive conditions are disestablished, said latch maintaining the tension during non-excessive local heating of the one thermostatic member.

7. A thermostat comprising a base, helical thermostats having ends in axial abutment, whereby ambient temperature changes affecting both thermostats result in no axial movement, said thermostats having between them a second torsional abutment comprising normally spaced apart switch contacts, manual means for driving one torsional abutment against the other while tensioning both thermostats, a latch for maintaining the tensioned condition in the thermostats and a closed condition of the switch contacts, means for heating one of the thermostats at a different rate than the other whereby movetorsionally, the torsional abutment constituting contacts normally open when said thermostatic elements are unstressed, a connection between said shaft and one thermostatic element to stress said thermostatic element and to close said contacts to stress the other thermostatic element when the shaft is moved, means for locally heating one thermostatic element whereby it expands to move the axial abutment, a latch for maintaining said contacts together under normal local l heating of the locally heated thermostat but to ,1,

release the torsional tension under abnormal heating, said latch again becoming potentially operative only after a cooling of the locally heated thermostat to substantially the conditions encountered under said normal local heating.

9. A thermostat comprising a manually rotary shaft, a spring tensioned by rotation of the shaft, a releasable latch for maintaining the rotated and tensioned condition, an axially movable abutment on the shaft and keyed thereto by a key, a switch arm rotary on the shaft, and a helical thermostat between said abutment and the switch arm adapted to expand axially to remove said abutment from said key against the spring upon temperature change in thefthermostatic element, the thermostat being under rotary control of the spring to open the switch after said removal.

10. A thermostat comprising a frame, a manually rotary shaft, a spring torsionally tensioned by rotation of the shaft, a latch for maintaining the tensionedl condition, manual means for releasing the latch, an axially movable abutment between said tensioned spring and the abutment whereby the abutment is torsionally moved by the spring after release to open said switch by the rotary connection between the abutment and the switch through-said helical thermostat.

11. A thermostat comprising a frame, a manually rotary shaft, a spring torsionally tensioned by rotation of the shaft, a latch for maintaining V the tensioned condition, manual means for releasing the latch, an axially movable abutment on the shaft and keyed thereto so as to permit said axial movement, a switch arm rotary on the shaft, and a helical thermostat between said abutment and the switch arm adapted to expand axially to remove said abutment from its keyed connection under excessive temperature change in the therinostatic element, and a connection between said tensioned spring and the abutment whereby the abutment is torsionally moved by the spring after release to open said switch by the rotary connection between-the abutment and the switch through said helical thermostat, said frame having a signal opening and means behind theopening indicating switch open position when the manual latch has been released, and means therebehind indicating switch closed position, said last-named means comprising an arm attached to the switch arm and positioned behind said opening when the switch lis closed.

12. A thermostat comprising a manually rotary shaft, a spring tensioned by rotation of the shaft, a releasable latch for maintaining the 'rotated and tensioned condition, anaxially movable abutment on the shaft and keyed thereto by a key, a switch arm rotary on the shaft, and two helical thermostats, one of said thermostats expanding and the other contracting upon increase in temperature, said one thermostat being adapted to expand axially to remove said abutment from said key against the spring upon temperature increase in said one thermostat, the

removal of said abutment permitting the spring to open the switch.

13. A thermostat comprising a helical coil composed of a strip of composite metal which is thermally responsive axially, stationary contacts, contacts operable by rotary movement with said coil to open and close an `electrical circuit through the contacts, the rotary movement of the opening and-closing of said contacts taking place at right angles to the direction of the thermally responsive motion of said coil.

14. A thermostat comprising al coil of composite metal wound in the form of a spring, said coil having axial movement in response to temperature change, a 'stationary contact, a contact rotary with said coil and cooperating with said stationary contact to open and close an electric circuit, means for torsionally stressing said coil, and latching means, said latching means being operable by said axial movement of said coil, and said contacts being operable by said torsional stress in said coil.

15. A thermostat consisting of a vthermostatic element comprising a strip of composite metal extending progressively about a minor axis to form minor coils, said minor axis being in turn extended about a second major axis to form major coils, a stop, a member mounted for predetermined motion in response to torsion of said thermostatic element about said major axis and against said stop, means for mechanically twisting the thermostatic element about the major axis to move said member against said stop and to introduce strain into the thermostatic element to hold said member xed, a latch adapted to close to maintain said strain after completion of `said twisting and responsive to open upon axial movement of the major coils relatively to one another to release strain in the thermostatic element whereby said member may move from its iixed position.

16. A thermostat consisting of a thermostatlc element comprising a strip of composite metal extending progressively about a rst minor axis to form minor coils, said minor axis being in turn extended about a second major axis to form major coils, a fixed contact and a movable contact mounted for motion of the latter toward former in response to rotation of said thermostatic element about said major axis, means for winding the thermostatic element about the major axis to move'said contact and introduce strain into the thermostatic element, a latch closing to maintain said strain after Winding and responsive to axial movement of the major coils relatively to one another to open to release strain in the thermostatic element Whereby the movable contact may move away from the xed contact.

17. A thermostat consisting of a thermostatic element comprising a strip of composite metal extending progressively about a minor axis to form minor coils, said minor axis being in turn extended about a second major axis to form major coils, means for placing said major coils under torsional stress, and means actuated by the axial movement of said thermostatic element in response to temperature change, and coacting with said rst-named means whereby said torsional stress is relieved.

18. A thermostat consisting of a thermostatic element comprising a strip of composite metal extending progressively about a minor axis to form minor coils, said minor axis being in turn extended about a second major axis to form major coils, means for placing said major coils under torsional stress by applying torsional strain, means coacting with said first-named means to maintain said stress at certain lower temperatures and releasing said stress inresponse to axial movement of the thermostatic element at certain higher temperatures.

WALDO A. SAUL. 

